Elastomers for Sustainability - any challengers to the current Top 10?

Entries are now being invited for the next Elastomers for Sustainability Top 10 programme, to be published in the July/August 2021 edition of European Rubber Journal magazine. 

To submit an entry for the next E4S Top 10 ranking, in ERJ July/Aug issue, please complete the online form by 19 March: .

For guidance and inspiration, over the coming weeks ERJ will publish details of the current Top 10 project entries.

Let's start here with ExxonMobil's impressive table-topping project entry: 

Company: ExxonMobil, Specialty Elastomers & Butyl Business    

Location: Spring, Texas, United States

Project: The Impact of Tire Pressure on Electric Vehicle (EV) Driving Range: 
Creating higher performance tire innerliners with new specialty elastomer to extend battery range  

Main materials technologies involved    

New Exxpro 3563 specialty elastomer to replace traditional halobutyl (HB)/natural rubber (NR) blends in tire innerliners. 

Exxpro is a class of fully saturated specialty elastomer produced by the carbocationic polymerization of isobutylene and para-methyl styrene (pMS) followed by halogenation, thereby completely eliminating isoprene.

Exxpro specialty elastomer has several advantages over halobutyl polymers, including a fully saturated backbone, which enables exceptional chemical and oxidation stability. The elastomer also has increased chain stiffness and better packing density with benzylic bromide groups in the chain.

This structure creates a much more tortuous path for oxygen molecules, enabling much lower permeability than halobutyl polymers. Since crosslinking occurs via substitution of the benzylic bromide groups, these systems can be cured sulfur-free.

Main goal or objective of the development project    

Improve the Inflation Pressure Loss Rate (IPLR) performance of tires for enhanced air retention and reduced rolling resistance to deliver better energy efficiency and extended battery range in EVs (a cause of anxiety for drivers). 

Technical challenges addressed by the project team    

Although electric vehicle manufacturers have made advances in extending battery range, most development has focused on battery technology, controls, and vehicle aerodynamics. Little attention has been given to tires, until now.  Tires lose a certain amount of air (typically 2.4% a month) on a continuous basis due to inefficient construction and sub-optimal inflation which affects EV range.  

An underinflated tire is less stiff and its deformation much higher, leading to a higher amount of heat dissipation, higher rolling resistance and poor range efficiency. In a recent study, which examined several production tires it was found that 48% had poor air retention (IPLR >3%). 

Only 6% had optimal air retention (IPLR < 1.7%).  It is difficult to design a tire that is entirely leak-proof, but it can air loss can be minimized by designing an effective tire innerliner, the thin layer responsible for air retention.  

Material composition and design of the innerliner are the most critical factors. Air loss is affected primarily by innerliner compound permeability, its thickness, and end-point-to-toe distance (the point where the innerliner ends). The biggest contributor is the compound permeability – reducing permeability by a coefficient of 40% delivers an IPLR improvement of 30%.  

Conventional tire innerliner compounds include bromobutyl and chlorobutyl rubber, but they do not deliver the best performance. Tests have proven that brominated isobutylene paramethylstyrene terpolymers (BIMSM), tradename Exxpro specialty elastomers, provide lower permeability than halobutyl rubber.

Because air loss over time reduces air pressure, the “in-use” rolling resistance experienced during actual driving conditions can be higher, leading to lower fuel economy/lower battery range. 

Air loss can be caused by excessive loads, high speeds, long idling times, aggressive driving (high acceleration and excessive braking), high aerodynamic drags (due to wind speed or cargo), cold weather, short travel distances, use of electrical accessories (such as the air conditioning system), driving on terrains/uphill routes and use of a four wheel drive . 

Laboratory tests for measuring rolling resistance coefficient would not necessarily capture these aspects. For example, internal ExxonMobil tests indicated that air loss in tires subjected to real world conditions was about twice that of static “in-laboratory” tests. So, a 6 month trial using “in-use” conditions was conducted. 

What is the commercial status of the technology or product?    

As a result of an “in-use” tire research study undertaken by Geely Auto Group (owners of Volvo), Linglong Tire Co. and ExxonMobil, Geely has established a new IPLR of 1.5 to 1.8% for all its NEV models.  Several manufacturers are conducting large scale trials using tires with innerliners made using Exxpro™ 3563 specialty elastomer.  

Please describe the contribution of the technology or product?    

With the radical changes in mobility happening now, maintenance-free tires with the lowest possible air-loss rates are needed more than ever. Innerliners made with Exxpro™ 3563 enable exceptional Inflation Pressure Loss Rate (IPLR) performance (< 1.8%) to deliver enhanced energy efficiency.

It also impacts the vehicle’s safety, emissions and extends tire life. The difference that can be measured with real data. Compared to the global average 80% halobutyl (HB)/ 20% natural rubber (NR) innerliner blend, Exxpro 3563 delivers: 
?    20-50% improved air retention
?    12% improvement in in-use rolling resistance
?    On target Inflation Pressure Loss Rate (IPLR) of <1.8% for global EV OEM leaders
?    Increase of 3-7% in EVs battery range

Because tires made with Exxpro specialty elastomer reduce the IPLR they can have less impact on EV driving range, enabling the industry to consider a lighter battery for cheaper VAVE.  Additionally, such high-performance tires can optimize the customer experience through a more consistent drive performance and lower maintenance frequency.

Scope for further enhancements to the technology or product?    

Exxpro 3563 has been commercialized

Any further comments to further highlight the contribution of this development project to environmental sustainability?    

To fully understand the difference between using tire innerliners made with Exxpro 3563 and those made with a conventional halobutyl rubber compound consider the following scenario.  

Assume an average vehicle travels 7,456 miles (12,000km) a year, the annual wasted energy difference between using these two sets of tires would amount to 36 kW-h. 

If you then consider that the total number of Chinese battery EV sales from 2012 to 2019 was about 4.97 million vehicles then using the tire with the better performing tire innerliner based on Exxpro would save almost 90 million kW of electricity every year. 

To put this into perspective it is the equivalent to the amount of electricity consumed by 50,000 Chinese households annually. Simply by using a tire with an innerliner made with Exxpro specialty elastomer (IPLR < 1.8%) and not halobutyl (IPLR 3.2%).

Additional information:.    

See Geely Technical Article in ERJ Technical Papers section, and check out relevant information on the ExxonMobil Chemical website.